Ultra-large area higher-order mode (HOM) fiber amplifiers have been successfully demonstrated. For example, by operating in the LP0,N mode of a specially-designed multi-mode fiber, amplifiers with effective areas (Aeff) of 6000 μm2 have been tested and shown to be suitable for high peak power pulse generation (e.g., peak power on the order of tens to hundreds of kW).
HOM fiber is known in the art and is generally comprised of optical fiber having a small inner core that guides the fundamental LP01 mode (and facilitates splicing the HOM fiber to conventional single mode fiber) and a larger outer core that guides the desired higher-order mode. Long-period gratings (LPGs) have been used to in conjunction with HOM fiber to shift an incoming signal propagating within a core region of a conventional fiber into the outer core region of the HOM fiber; that is, converting the mode of the incoming signal from the fundamental LP01 mode to a higher-order, selected LP0,N mode. This configuration has been found useful in the formation of doped fiber amplifiers, where a section of HOM fiber is fabricated to include a selected rare-earth dopant, and then used to perform optical amplification on the higher-order mode form of the propagating optical signal.
Since an amplified signal beam with a low M2 value is typically required (where M2 is a well-known beam quality measure that describes the deviation of the propagating beam from a theoretical Gaussian beam; in applications such as those of the present invention where the goal is to create a diffraction-limited beam, a value of M2 as close to unity as possible is therefore desired), conversion of the amplified higher-order mode signal into its fundamental mode is typically used to create the desired diffraction-limited output beam. A second LPG is typically used to provide this mode re-conversion at the output of the HOM fiber amplifier.
While an output LPG for mode re-conversion is convenient, at high peak power (e.g., tens to a few hundred kW), a conventional fiber-based LPG device has been found to exhibit nonlinearities in the form of self-phase modulation in the presence of high power signals. These nonlinearities alter the mode conversion properties of an LPG in an unwanted fashion. At even higher peak powers (e.g., hundreds of kW to MW), permanent changes in the physical properties of LPGs have been observed. These nonlinear effects thus significantly impact the ability to generate the required diffraction-limited beams at the output of high peak power HOM amplifiers.
While these undesirable nonlinear effects can be somewhat mitigated by carefully controlling the design and properties of the HOM fiber and LPGs, it is not always practical to do so. For example, the fundamental mode area (i.e., the inner core) of the HOM fiber can be made larger, and/or the LPG can be made stronger and shorter (or possibly chirped). However, imposing the condition that the output beam must be spatially compressed into the small effective area fundamental mode of an LPG will ultimately lower the peak power-handling capability of an HOM amplifier when compared to an amplifier that does not require this spatial compression.
For at least these reasons, alternative mode conversion strategies that do not require re-entering small effective area fundamental mode are sought after for high peak power amplifiers.